Spectral sensitization is a very important, essential technique in the preparation of high-sensitivity and stable silver halide photographic materials. Many spectral sensitizers have been developed, and the practical application methods of them with regard to, for example, supersensitization methods, addition methods, have been developed.
Conventional spectral sensitizing dyes used in conducting spectral sensitization include spectral sensitizers such as cyanine dyes, merocyanine dyes and rhodacyanine dyes. It is known that these spectral sensitizers are used either alone or in combination (e.g., for the purpose of supersensitization).
The sensitizing dyes used in photographic materials must meet many requirements. For example, not only high spectral sensitivity can be provided, but also fog is not to be increased. It is necessary that the sensitizing dyes have excellent exposure characteristics (e.g., latent image stability, reciprocity law characteristics, temperature or humidity dependency during exposure), hardly cause a change in sensitivity, gradation and fog during the storage of the photographic materials before exposure and are not left behind in the photographic materials after processing.
Particularly, it is an essential factor that the sensitizing dyes are highly stable during the storage of the photographic materials. Many attempts have been made to provide sensitizing dyes satisfying the above requirements.
Conventionally, photographic emulsions containing oxacarbocyanine dyes alone (e.g., described in U.S. Pat. Nos. 2,173,486, 2,521,706 and 3,044,875, and JP-A-63-167348 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), photographic emulsions containing a combination of oxacarbocyanine dyes and imidadicarbocyanine dyes (e.g., described in JP-A-59-116646, JP-A-59-116647, JP-A-59-140443 and JP-A-59-149346) and photographic emulsions containing a combination of two or more of oxacarbocyanine dyes are known as silver halide photographic emulsions having spectral sensitivity in the wavelength region of green light. However, the storage stability thereof has not been sufficient.
Further, it is important that various dyes are added to the silver halide photographic materials to improve sharpness and an ability of color separation.
Supersensitization is described in Photographic Science and Engineering, vol. 13, pp. 13-17 (1969), ibid. vol. 18, pp. 418-430 (1974) and James, The Theory of the Photographic Process, the fourth edition, page 259 (Macmillan 1977). It is known that high sensitivity can be obtained by selecting appropriate sensitizing dyes and supersensitizing dyes.
Dyes having a benzoxazole nucleus having a substituted aryl group at the 5-position of the nucleus are disclosed in U.K. Patent 675,654. The dyes have such a structure having a phenyl group at the 5-position, and an alkoxy group or an alkyl group at the para-position of the phenyl group as follows: ##STR2##
Namely, a dye having a benzoxazole nucleus substituted at the 5-position of the nucleus by a halogen-substituted aryl group at the para-position of the aryl group has not been conventionally known.